Arc-extinguishing materials

ABSTRACT

Arc-extinguishing materials are selected in accordance with criteria such as first ionization potential of each of the constituents, carbon content, boiling, sublimation, or decomposition temperature, endothermic character of decomposition, electronegativity of decomposition products, rate of reformation, dimensional and chemical stability, ease of application, cost, toxicity, and coefficient of thermal conductivity.

United States Patent [191 Khalid 1 Dec. 30, 1975 ARC-EXTINGUISHINGMATERIALS [75] Inventor: Joseph M. Khalid, Cedar Rapids,

Iowa

[73] Assignee: Square D Company, Park Ridge, 111.

[22] Filed: May 29, 1973 [21] Appl. No.: 364,596

OTHER PUBLICATIONS Bailar et a1., Comprehensive Inorganic Chem, 1973,pp. 880-882.

Dove 252/63.5

Hawley, Condensed Chemical Dictionary, 8th Ed, (1971).

Grant, Hackhs Chemical Dictionary, 4th Ed, (1969).

The Merck Index, (1968), 8th Ed.

Primary Examiner-Leland A. Sebastian Assistant Examiner-B. HuntAttorney, Agent, or Firm-Harold J. Rathbun; Ernest S. Kettelson [57]ABSTRACT Arc-extinguishing materials are selected in accordance withcriteria such as first ionization potential of each of the constituents,carbon content, boiling, sublimation, or decomposition temperature,endothermic character of decomposition, electronegativity ofdecomposition products, rate of reformation, dimen sional and chemicalstability, ease of application, cost, toxicity, and coefficient ofthermal conductivity.

9 Claims, N0 Drawings ARC-EXTINGUISHING MATERIALS This invention relatesto arc-extinguishing materials usable as molding compositions formolding arc shields, arc chute frame components, cases of electricaldevices, and the like and also usable in coating formfor coating surfaceportions of a member wholly or partly defining an arc chamber in whichseparable contacts of an electrical circuit interrupting devicearedisposed.

Arc interruption requires the establishment of conditions promoting ahigher rate of recombination than the rate of ionization. The problemreduces to one of recognizing the basic factors that control these tworates. 1.

One of the factors which should be considered in the selection ofarc-extinguishing materials is the first ionization potential of each ofthe constituents of the material. The first ionization potential of anelement is the amount of energy required to remove one electron away toinfinity from a neutral atom of that element. For silver, the firstionization potential is 7.54 electron volts, while the first ionizationpotential for aluminum is 5.98 electron volts. To appreciate thesignificance of a difference of one electron volt between the firstionization potentials of two elements, consider that the temperaturedifference between two particles having an energy difference of oneelectron volt is 7730 C.

Another factor which should be considered in the selection ofarc-extinguishing materials is electronega: tivity of decompositionproducts. This is a measure of the affinity of an atom of an element forelectrons. .As an arc current approaches zero it would be desirable tohave an electronegative gas in the arc chamber that would capture thecooling and slowing electrons. Such electrons would otherwise beaccelerated by the rising recovery voltage and cause further ionizationand possible reignition.

Yet anotherfactor which should be considered in the selection ofarc-extinguishing materials is the boilingsublimation, or decompositiontemperature. During the few microseconds preceding and following thepassing of the current through zero in an alternating current system,the arc column temperature decreases rapidly until it reaches thetemperature of the arc chamber walls, which actas a constant temperaturesink. The higher the boiling temperature of the material of the arcchamber walls is, the lower the recombination rates will be.

Endothermic processes have their primary importance at current zero,particularly in the mode of arc interruption wherein a post-arc currentflows. Under this latter condition the energy being absorbed in anendothermic process taking place in the arc chamber on a modest scalemay approximate the energy being put into the chamber by the post-arccurrent. This effect, complemented by a favorable boiling temperature,significantly contributes to the success of arc extinction.

lf an arc-extinguishing material decomposes to yield free carbon, thiscarbon may be oxidized to carbon dioxide. The process is exothermic, andits timing is bad, because it is as the arc current approaches zero andthe temperature drops and recombination of ions and dissociatedmolecules starts to take place that the exothermic formation of carbondioxide occurs. Deposits of free carbon can also cause tracking anddielectric failure. However, to limit arc-extinguishing materials tothose containing no carbon would too severely limit the choice ofmaterials. Moreover, hydrogenhas been found to be a desirable componentof the gas medium surrounding an arc, especially when an applicationmakes it desirable to force motion of the arc by' a' transverse magneticfield, and carbon and hydrogen'generally occur together. It has beenfound that the number of carbon atoms as a percentage of the totalnumber of atoms in an arc-extinguishing material should not exceed avalue of about fifteen percent.

An endothermic process of decomposition is exothermic on'formation.Therefore, it is desirable to select materials having formulas such thatthe probability of reformationafter decomposition is small. Aluminumoxide (Al O and boron oxide (B 0 are materials with such formulas,because the. probability of two aluminum or baron atoms collidingsimultaneously with three oxygen atoms, or with one molecule and oneatom of oxygen, is small. However, aluminum oxide is undersirable as anarc-extinguishing material because the ionization potential of aluminumis low and the boiling temperature ofaluminum oxide is too high. Inthis'respec't=,-the present invention is a departure from the prior" artas represented by U.S. Pat. Nos.

- 2,768,264, and 3,071,666 which tout aluminum oxide as a goodarc-extinguishing material.

SUMMARY OF CRITERIA In accordance with the invention, the firstionization potential of each of the constituents of a suitablearcextinguishing material should be equal to or greater than 7.54electron volts, which is the first ionization potential of silver. Theatomic concentration of carbon in the material should not be greaterthan about 15 per cent. The boiling, sublimation, ordecompositiontemperatu're of thematerial should be as low as possible consistent withother requirements, preferably below 2,000 C. Decomposition of thematerial should be a strongly endothermic process, the more endothermicthe better. The products of decomposition of the material should be aselectronegative as possible consistent with other requirements. Thematerial should have a formula such that the rate of reformation afterdecomposition is vanishingly small near the zero point of thealternating current, such as that for B 0, or H BO The material shouldbe non-toxic, non-caustic, noninflammable, easyto process and apply, anddimensionally and chemically stable under operating conditions. Further,it should have good thermal conductivity for an electrically insulatingmaterial, preferably greater than 0.001 calories per second per squarecentimeter per degree Centigrade per centimeter.

Representative Materials In general the oxides, borides, borates,silicates, and the ammonium complexes of the elements having a firstionization potential equal to or greater than that of silver (7.54eV)are suitable arc-extinguishing materials, and so are their hydratedforms. A preferred selection of such elements, with their firstionization potentials shown in parentheses, is tantalum (7.70eV), copper(7.72eV), cobolt (7.86eV), rhenium (7.87eV), iron (7.90eV), tungstem(7.98eV), silicon (8-.l5eV), boron (8.29eV), palladium (8.30eV),antimony (8.64eV), tellurium (9.0leV), zinc (9.39), and selenium(9.75eV). These substances may also be used as fillers in suitableresins provided that the atomic carbon concentration does not exceed thenominal limit of fifteen percent. Silicone resins are suitable. The useof 3 resins becomes mandatory when the filler material cannot be used inits pure form. For example, pure silicon dioxide cannot be used becauseits boiling tem-v perature is too high, and pure boric acid cannot beused because it is water soluble and therefore dimensionally unstable,but these materials are suitable when used in suitable resins.

Beryllium (9.32eV), arsenic (9.8leV), and mercury (10.43eV) have firstionization potentials higher than that of silver, but these elements aretoxic and therefore undesirable. Sulphates, phosphates, nitrates, andhalogens are also undesirable as toxic.

Silver, platinum, and gold oxides, borates and silicates, with orwithout ammonium complexes, would be suitable arc-extinguishingmaterials except for high cost.

In particular, a material composed of 55.8 per cent by weight FirebrakeZ8 and 44.2 per cent Sylgard 182 has been found to be a very suitablearc-extinguishing material. Firebrake ZB is made by United States Boraxand Chemical Corporation of Los Angeles, Calif., and has a formula2ZnO.3B O .3/l-l O. Sylgard 182 is made by Dow Corning Corporation ofMidland, Mich. and is a dimethyl silicone resin having a basic monomerrepresented by the following structural formula:

SiO

I claim:

1. An arc-extinguishing material consisting of a compound comprising aprimary ingredient having arcextinguishing properties, and a binder,said primary ingredient including an element having a first ionizationpotential no less than 7.54 electron volts and selected from a classconsisting of silver, tantalum, copper, rhenium, tungsten, boron,palladium, tellurium, zinc and selenium, the relative proportions ofsaid primary ingredient and said binder being a silicone resin, thecarbon atoms of said total compound being no greater than fifteenpercent of the total concentration of atoms thereof.

2. An arc-extinguishing material as set forth in claim 1, wherein saidcompound has a boiling temperature no greater than 2,000 C, and saidelement is selected 4 from a class consisting of silver, tellurium, zincand selenium, the boiling points of such elements in pure form beingless than 2,000 C.

3. An arc-extinguishing material as set forth in claim 1, wherein saidcompound has a boiling temperature no greater than 2,000 C, and saidelement is selected from a class consisting of tantalum, copper,tungsten, boron and palladium, the boiling points of such elements inpure form being greater than 2,000 C, said binder and said selectedelement comprising said compound having a boiling temperature no greaterthan 2,000 C.

4. An arc-extinguishing material consisting of a compound characterizedby a negligible rate of reformation after decomposition, said compoundbeing boron oxide and a silicon based resin.

5. An arc-extinguishing material consisting of a compound characterizedby a negligible rate of reformation after decomposition, said compoundbeing boric acid and a silicone based resin.

6. An arc-extinguishing material as set forth in claim 1, wherein saidprimary ingredient comprises at least 40 percent by weight of saidcompound.

7. An arc-extinguishing material as set forth in claim 1, wherein saidprimary ingredient comprises at least 55 percent by weight of saidcompound.

8. An arc-extinguishing material as set forth in claim 1, wherein saidprimary ingredient comprises about 56 percent by weight of saidcompound.

9. An arc-extinguishing material consisting of a compound characterizedby negligible rate of reformation after decomposition, a firstionization potential no less than 7.54 electron volts, a boilingtemperature no greater than 2,000 C, for which decomposition is anendothermic process, having thermal conductivity of at leastone-thousandth of a calorie per second per square centimeter per degreeCentigrade per centimeter, said compound comprising a primary ingredientand a binder, said primary ingredient including an element selected froma class consisting of silver, tantalum, copper, rhenium, tungsten,boron, palladium, tellurium, zinc and selenium, said binder being asilicone resin, the carbon content and proportion thereof relative tosaid primary ingredient being such that the carbon atoms are no greaterthan 15 percent of the total atoms of said compound.

1. AN ARC-EXTINGUISHING MATERIAL CONSISTING OF A COMPOUND COMPRISING APRIMARY INGREDIENT HAVNG ARC-EXTINGUISHING PROPERTIES, AND A BINDER,SAID PRIMARY INGREDIENT INCLUDING AN ELEMENT HAVING A FIRST IONIZATIONPOTENTIAL NO LESS THAN 7.54 ELECTRON VOLTS AND SELECTED FROM A CLASSCONSISTING OF SILVER, TANTALUM, COPPER, RHENIUM, TUNGSTEN, BORON,PALLADIUM, TELLURIUM, ZINC AND SELENIUM, THE RELATIVE PROPORTIONS 0FSAID PRIMARY INGREDIENT AND SAID BINDER BEING A SILICONE RESIN, THECARBON ATOMS OF SAID TOTAL COMPOUND BEING NO GREATER THAN FIFTEENPERCENT OF THE TOTAL CONCENTRATION OF ATOMS THEREOF.
 2. Anarc-extinguishing material as set forth in claim 1, wherein saidcompound has a boiling temperature no greater than 2,000* C, and saidelement is selected from a class consisting of silver, tellurium, zincand selenium, the boiling points of such elements in pure form beingless than 2,000* C.
 3. An arc-extinguishing material as set forth inclaim 1, wherein said compound has a boiling temperature no greater than2,000* C, and said element is selected from a class consisting oftantalum, copper, tungsten, boron and palladium, the boiling points ofsuch elements in pure form being greater than 2,000* C, said binder andsaid selected element comprising said compound having a boilingtemperature no greater than 2,000* C.
 4. An arc-extinguishing materialconsisting of a compound characterized by a negligible rate ofreformation after decomposition, said compound being boron oxide and asilicon based resin.
 5. An arc-extinguishing material consisting of acompound characterized by a negligible rate of reformation afterdecomposition, said compound being boric acid and a silicone basedresin.
 6. An arc-extinguishing material as set forth in claim 1, whereinsaid primary ingredient comprises at least 40 percent by weight of saidcompound.
 7. An arc-extinguishing material as set forth in claim 1,wherein said primary ingredient comprises at least 55 percent by weightof said compOund.
 8. An arc-extinguishing material as set forth in claim1, wherein said primary ingredient comprises about 56 percent by weightof said compound.
 9. An arc-extinguishing material consisting of acompound characterized by negligible rate of reformation afterdecomposition, a first ionization potential no less than 7.54 electronvolts, a boiling temperature no greater than 2,000* C, for whichdecomposition is an endothermic process, having thermal conductivity ofat least one-thousandth of a calorie per second per square centimeterper degree Centigrade per centimeter, said compound comprising a primaryingredient and a binder, said primary ingredient including an elementselected from a class consisting of silver, tantalum, copper, rhenium,tungsten, boron, palladium, tellurium, zinc and selenium, said binderbeing a silicone resin, the carbon content and proportion thereofrelative to said primary ingredient being such that the carbon atoms areno greater than 15 percent of the total atoms of said compound.